At least 27 people including a dozen schoolchildren are dead after torrential rains triggered flash floods and landslides on the Indonesian island of Sumatra, officials said.

Key points:

Almost 30 children swept away when flash flood hits school

More than 500 homes damaged and three bridges destroyed

Rescuers struggle to reach mountainous areas due to damaged roads

A flash flood with mud and debris from landslides struck Mandailing Natal district in North Sumatra and smashed an Islamic school in Muara Saladi village, where 29 children were swept away on Friday afternoon, said local police chief Irsan Sinuhaji.

He said rescuers retrieved the bodies of 11 children from mud and rubble hours later.

"The victims were buried in a torrent of mud and wall debris," said National Disaster spokesman Sutopo Purwo Nugroho.

Rescuers and villagers managed to rescue 17 other children and several teachers on Friday and pulled out the body of a child on Saturday near Aek Saladi river, close to the school.

Footage showed relatives crying besides their loved ones at a health clinic where the bodies of the children were lying, covered with blankets.

More than 500 homes in the provinces of North and West Sumatra have been flooded or damaged and three suspension bridges were destroyed.

"Evacuation as well as search and rescue operations are underway," Mr Nugroho said.

"But the affected villages are in the mountains and access is difficult, due to damaged roads."

Two people were found dead on Saturday after their vehicles were swept away by the river.

Four people died in landslides in the city of Sibolga in North Sumatra, while flash floods in West Sumatra killed at least four more, including two children.

Both North and West Sumatra provinces declared a week-long emergency relief period as hundreds of terrified survivors fled their hillside homes to safer ground, fearing more of the mountainside would collapse under continuing rain.

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